Routing loop

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A routing loop occurs when a distributed path selection mechanism creates a (usually temporary) loop in the path to a particular destination; i.e. the routing table entries in a group of switching nodes, with respect to given destination, form a closed loop. Any traffic for that destination that enters that loop never leaves - just loops endlessly until the loop is broken.

Routing loops generally form as a result of out-dated data being used by some of the switching nodes in the loop, when doing their path selection computations - e.g. after another switching node has failed, and paths which previously went through it are being re-computed. They generally do not last long, and disappear as updated data arrives.

Even so, they are a serious problem while they are there, and a number of mechanisms (such as Path Vector routing protocols) have been devised to try and avoid their formation.